1. CHE 112 MODULE # 3 Chapter 16 The Nature of Chemical Equilibrium

2. Equilibrium  Equilibrium is a state in which both the forward and the reverse reactions continue at equal rates with no observable change.  Use balanced equations with a double arrow  to symbolize the forward and reverse reactions.

3. Chemical Equilibria  Chemical reactions are dynamic and reversible. CH 3 COOH + H 2 O  CH 3 COO - + H 3 O + acetic acid acetate ion hydronium ion This weak acid reaction will be studied in detail in the next few chapters.

4. Equilibrium Constant  For the reaction: aA + bB  cC + dD  The equilibrium constant K c is expressed as K c = [C] c [D] d [A] a [B] b  K c should be the same for all experiments performed at the same temperature.

5. K c for HI  Given the following reaction at 425°C (pg658): H 2(g) + I 2(g)  2HI (g) K c = [HI] 2 = 56 [H 2 ][I 2 ] No matter what the initial concentration of the reaction mixture is the K c will remain the same.

6. K c for the Sum of Reactions  Equilibrium constants for a sum of reactions are multiplied together: CO + 3H 2  CH 4 + H 2 O; K c1 = 3.92 CH 4 + 2H 2 S  CS 2 + 4H 2 ; K c2 = 3.3x10 4 CO + 2H 2 S  CS 2 + H 2 O + H 2 K c = K c1 K c2 = (3.92)(3.3x10 4 ) = 1.3x10 5

7. Closer Look at K c  Homogenous Equilibrium – Rxn where products and reactants are in a single phase  Heterogeneous Equilibrium – Rxn where products and reactants are in more than one phase CaCO 3(s)  CaO (s) + CO 2(g) Where K c = [CO 2 ]

8. Heterogeneous Equilibrium  Given the decomposition of water, what is the equilibrium expression for this reaction? 2H 2 O (l)  2H 2(g) + O 2(g) K c = [H 2 ] 2 [O 2 ]

9. Purpose of K c  By it’s magnitude we can tell whether the reaction mixture contains more product, more reactant, or if they are about the same in concentration. (Think fractions)  If K c is large, then the mixture is mostly product  If K c is small, then the mixture is mostly reactant  If K c is about 1, the products  reactants

10. Purpose of K c (cont.)  For reactions not at equilibrium, we can determine the reaction quotient (Q c ) and compare it to the K c to see if the reaction is product or reactant favored.  If Qc > Kc, the reaction favors the reactants  If Qc < Kc, the reaction favors the products  If Qc = Kc, the reaction is at equilibrium

11. Equilibrium Concentrations  If we know the K c, we can back calculate to determine the equilibrium concentration from any set of starting concentrations. CO + 3H 2  CH 4 + H 2 O; K c = 3.92 If at equilibrium the concentrations are 0.3M CO, 0.1M H 2, and 0.02M H 2 O @1200K, then K c = [CH 4 ][H 2 O], 3.92 = [CH 4 ](0.020), [CO][H 2 ] 3 (0.3)(0.1) 3 [CH 4 ] =0.06M

12. Calculating with K c  K c = 4.00x10 -4 for the following reaction: N 2(g) + O 2(g)  2NO (g) Given: The equilibrium concentrations of nitrogen and nitrogen monoxide are 3.00x10 -2 M and 1.00x10 -3 M. Calculate the concentration of oxygen at equilibrium.

13. Factors that Upset Equilibria  The factors that can affect the equilibrium of a reaction system are: Adding or removing a substance in the reaction Changing the volume of the reactant vessel Changing the temperature Changing the pressure of the system

14. Le Chatelier’s Principle  When a reaction at equilibrium is disturbed by a change in concentration of one of the reactants or products, volume, temperature, and even pressure the reaction will shift to compensate or counteract the change in order to reestablish the system to equilibrium.

15. LeChatelier’s Principle Activity  Review the following screens on your CD: Screen 16.11 – outline & description Screen 16.12 – outline, description & tutorial Screen 16.13 – outline, description (pages 1& 2), simulation, and tutorial Screen 16.14 – outline, description, and simulation

16. LeChatelier’s Principle Activity  Because chemicals containing dichromate are very toxic, we no longer do this classic equilibrium lab. But you can follow it through (with pictures) by going to the website: http://www.carlton.paschools.pa.sk.ca/chemical/e quilibrium/dichromate/LCP_procedureP.htm